I have posted some of this in other places, not sure if Larian even looks here.
So what comes next after BG3? I have a thought that is a bit outside the box. I want to see Larian gets the rights to use the Dangerous Journeys game system. This system was what Gygax envisioned as the replacement for DnD. The system is skilled based with a deep and ROBUST magic system. Player freedom is amazing and the game system can be adapted to any setting.
I would ignore the Aerth game world that was bundled with the system and instead build a whole new game world. While I would love to see Greyhawk or more Forgotten Realm adventures, I can understand working with WotC is a bit of an issue.
Dangerous Journeys gives the game and excellent RPG system that will adapt to PC easily and should work very well with the existing engine Larian has developed. The open nature of the RPG system means they would not have to rework the system with each new game, giving them a solid frame work to work off of for at least a decade or more.